The present invention relates to probe measurement systems for measuring the electrical characteristics of integrated circuits or other microelectronic devices at high frequencies that extend into the gigahertz range.
There are many types of probing assemblies that have been developed for the measurement of integrated circuits and other forms of microelectronic devices. One representative type of assembly uses a circuit card on which are formed elongate conductive traces that serve as signal and ground lines. A central opening is formed in the card, and a needle-like probe tip is attached to the end of each trace adjacent the opening so that a radially extending array of downwardly converging needle-like tips is presented by the assembly for selective connection with the closely spaced pads of the microelectronic device being tested. A probe assembly of this type is shown, for example, in Harmon U.S. Pat. No. 3,445,770. This type of probing assembly, however, is unsuitable for use at higher frequencies, including microwave frequencies in the gigahertz range, because at such frequencies the needle-like tips act as inductive elements and because there are no adjoining elements present to suitably counteract this inductance with a capacitive effect in a manner that would create a broadband characteristic of more or less resistive effect. Accordingly, a probing assembly of the type just described is unsuitable for use at microwave frequencies due to the high levels of signal reflection and substantial inductive losses that occur at the needle-like probe tips.
In order to obtain device measurements at somewhat higher frequencies than are possible with the basic system just described, various related probing systems have been developed. Such systems are shown, for example, in Evans U.S. Pat. No. 3,849,728, Kikuchi Japanese Publication No. 1-209,380, Sang et al. U.S. Pat. No. 4,749,942, Lao et al. U.S. Pat. No. 4,593,243, and Shahriary U.S. Pat. No. 4,727,319. Yet another related system is shown in Kawanabe Japanese Publication No. 60-223,138 which describes a probe assembly having needle-like tips where the tips extend from a coaxial cable-like structure instead of a probe card. A common feature of each of these more recent systems is that the length of the isolated portion of each needle-like probe tip is limited to the region immediately surrounding the device-under-test in order to minimize the region of discontinuity and the amount of inductive loss. However, this approach has resulted in only limited improvement in higher frequency performance due to various practical limitations in the construction of these types of probes. In Lao et al., for example, the length of each needle-like tip is minimized by using a wide conductive blade to span the distance between each tip and the supporting probe card, and these blades, in turn, are designed to be arranged relative to each other so as to form transmission line structures of stripline type. As a practical matter, however, it is difficult to join the thin vertical edge of each blade to the corresponding trace on the card while maintaining precisely the appropriate amount of face-to-face spacing between the blades and precisely the correct pitch between the ends of the needle-like probe tips.
One type of probing assembly that is capable of providing a controlled-impedance low-loss path between its input terminal and the probe tips is shown in Lockwood et al. U.S. Pat. No. 4,697,143. In Lockwood et al., a ground-signal-ground arrangement of strip-like conductive traces is formed on the underside of an alumina substrate so as to form a coplanar transmission line on the substrate. At one end, each associated pair of ground traces and the corresponding interposed signal trace are connected to the outer conductor and the center conductor, respectively, of a coaxial cable connector. At the other end of these traces, areas of wear-resistant conductive material are provided in order to reliably establish electrical connection with the respective pads of the device to be tested. Layers of ferrite-containing microwave absorbing material are mounted about the substrate to absorb spurious microwave energy over a major portion of the length of each ground-signal-ground trace pattern. In accordance with this type of construction, a controlled high-frequency impedance (e.g., 50 ohms) can be presented at the probe tips to the device under test, and broadband signals that are within the range, for example, of DC to 18 gigahertz can travel with little loss from one end of the probe assembly to another along the coplanar transmission line formed by each ground-signal-ground trace pattern.
Although the probing assembly shown in Lockwood et al. provides satisfactory electrical performance at microwave frequencies, in the intervening period of time since this probe was first introduced, there has been a shift in emphasis in microwave probing technology toward high-volume production-oriented measurements. In accordance with this shift, there has been increasing demand for a probing assembly that can be rapidly positioned accurately on the closely-spaced contact pads of a microelectronic device and that, at the same time, is sufficiently rugged to withstand repeated use in a high-volume production environment without undue wear or component failure. With respect to the Lockwood et al. probing assembly, however, the opacity of the substrate tends to contribute to delay in the positioning process since the operator's view of the contact pads on the device is obscured at least part of the time by the substrate as the operator shifts the substrate back-and-forth over the pads while attempting to align the contacting areas on the substrate's underside with the pads. There has also been increasing demand for a probing assembly that has the capacity to probe non-planar as well as planar surfaces. This feature not only enables the measurement of such devices as multi-chip modules (MCMs), fired ceramic substrates, and flip-chips, but can also compensate for nonideal measurement conditions such as can occur, for example, when the device pads differ in height or when a loose metallic particle of minute dimension adheres electrostatically to the surface of one of the pads of the device-under-test so as to form a non-planar surface irregularity or when the plane of the device-under-test is inadvertently tilted at a slight angle in relation to the plane of the coplanar tips of the probing assembly. There is no specific mechanism on the Lockwood et al. probing assembly, however, that would enable the contacting areas on the substrate's underside to spatially conform to a non-planar arrangement of pads or other contact surfaces.
To achieve improved spatial conformance between the tip conductors of a probe and an array of non-planar device pads or surfaces, alternative high-frequency probing assemblies have been developed. Such assemblies are described, for example, in Drake et al. U.S. Pat. No. 4,894,612, Coberly et al. U.S. Pat. No. 4,116,523 and Boll et al. U.S. Pat. No. 4, 871,964. As in Lockwood et al., the Drake et al. probing assembly includes a substrate on the underside of which are formed a plurality of conductive traces which collectively form a coplanar transmission line. However, in one embodiment shown in Drake et al., the tip end of the substrate is notched so that each trace extends to the end of a separate tooth and the substrate is made of moderately flexible nonceramic material. The notches between the teeth provide a marginal degree of improvement in the visibility of the device pads during positioning of the probe tip, and the moderately flexible substrate permits, at least to a limited extent, independent flexure of each tooth relative to the other teeth so as to enable spatial conformance of the trace ends to slightly non-planar contact surfaces on a device-under-test. However, the Drake et al. probing assembly is of insufficiently rugged design to be satisfactory for use in high-volume production-oriented environments. Repeated flexing of the substrate teeth, for example, fatigues the supporting substrate and causes minute dislocations to gradually form within the substrate material. This eventually leads to significant degradation in the high-frequency performance of the Drake et al. assembly. Likewise, the probing assembly shown in Coberly et al. is ultimately susceptible to significant performance degradation due to the gradual formation of dislocations in the Mylar.TM. material that encapsulates the center finger or vertical blade of that particular assembly.
With respect to the probing assembly shown in Boll et al., as cited above, the ground conductors comprise a pair of leaf-spring members the rear portions of which are received into diametrically opposite slots formed on the end of a miniature coaxial cable for electrical connection with the cylindrical outer conductor of that cable. The center conductor of the cable is extended beyond the end of the cable (i.e., as defined by the ends of the outer conductor and the inner dielectric) and is gradually tapered to form a pin-like member having a rounded point. In accordance with this construction, the pin-like extension of the center conductor is disposed in spaced apart generally centered position between the respective forward portions of the leaf-spring members and thereby forms, in combination with these leaf-spring members, a rough approximation to a ground-signal-ground coplanar transmission line structure. The advantage of this particular construction is that the pin-like extension of the cable's center conductor and the respective forward portions of the leaf-spring members are each movable independently of each other so that the ends of these respective members are able to establish spatially conforming contact with any non-planar contact areas on a device being tested. On the other hand, the transverse-spacing between the pin-like member and the respective leaf-spring members will vary depending on how forcefully the ends of these members are urged against the contact pads of the device-under-test. In other words, the transmission characteristic of this probing structure, which is dependent on the spacing between the respective tip members, will vary in an ill-defined manner during each probing cycle.
Another difficulty with the Boll et al. probing structure is that the pin-like member and the leaf-spring members develop quite different levels of contact force when pressed against the corresponding contact pads of the device being tested. This nonuniformity of contact force can cause uneven wearing of the pads and the probing members or even more substantial damage. In order to partially alleviate this problem, a downwardly curving bend is formed in each of the probing members, and the leaf-spring members, in particular, are so arranged that their respective ends extend to a lower plane than the end of the pin-like member. Accordingly, the more flexible leaf-spring members are able to develop a moderate level of contact pressure against the corresponding pads on the device before the stiffer pin-like member engages its corresponding pad. However, the respective amounts of contact force exerted by the pin-like member and the leaf-spring members are still likely to differ significantly from each other since no mechanism is provided for deflecting each member by a set amount during each probing cycle.
In accordance with the foregoing, then, an object of the present invention is to provide an improved probing assembly that is operable at microwave frequencies extending into the gigahertz range and that is capable of providing, at such frequencies, a transmission channel of well-defined impedance for low-loss transfer of signals to and from the contact pads of a device-under-test.
A related object of the present invention is to provide a probing assembly of the above general type that can be used to probe non-planar surfaces with minimal degradation of electrical performance.
Another related object of the present invention is to provide a probing assembly of the above general type that affords good visibility of the closely-spaced contact pads of a device-under-test so that the tips of the assembly can be rapidly positioned accurately on such pads.
Yet another related object of the present invention is to provide a probing assembly of the above general type that is sufficiently rugged to withstand repeated use in a high-volume production environment without undue performance degradation or component failure.